1. Field of the Invention
The present invention relates to a controller. More particularly, the present invention relates to a driving circuit for the controller.
2. Description of Related Art
With the progress of modern science, the functionality of the electronic devices becomes better and better for people it is more the convenience in the living. Nowadays, greater part of the electronic devices include a controller to generate a driving signal for control other circuits of the electronic devices, such as power supply, power converter and regulator, etc.
FIG. 1 shows a circuit diagram of a traditional power supply. As shown in the figure, the traditional power supply comprises a transformer 10 having a primary winding NP, a secondary winding NS and an auxiliary winding NA. A terminal of the primary winding NP is coupled to receive an input voltage VIN. A transistor 12 is coupled from another terminal of the primary winding NP to a terminal of a current sense resistor 14. Another terminal of the current sense resistor 14 is coupled to the ground. The current sense resistor 14 is used to convert a switching current IP of the transformer 10 to a sense voltage VI. A start resistor 16 is connected from the input voltage VIN to charge a supplied capacitor 18. The supplied capacitor 18 further is connected to an input terminal VDD of a control circuit 35 of a controller 30 for supplying the power to the control circuit 35. Once the voltage VDD is charged up to a start up voltage of the control circuit 35, the control circuit 35 will start to operate.
A ground terminal GND of the control circuit 35 is coupled to the ground. A current sense terminal VS of the control circuit 35 receives the sense voltage VI which represents the switching current IP of the transformer 10. A feedback terminal FB of the control circuit 35 is coupled to receive an output voltage VO at an output terminal of the power supply through an optical-coupler 20 to generate a feedback signal VFB in response to the output voltage VO. In accordance with the feedback signal VFB and the sense voltage VI, a switching signal VG is generated by an output terminal VG of the control circuit 35 and then transmitted to a driving circuit 38 of the controller 30. In accordance with the switching signal VG, the driving circuit 38 generates a driving signal VD to turn on/off the transistor 12 for switching the transformer 10 and regulating the output voltage VO of the power supply. After the switching of the transformer 10, the power of the control circuit 35 further is supplied from the auxiliary winding NA of the transformer 10 via a diode 19. If a fault condition is occurred, the switching of the transformer 10 will be stop and the supplied capacitor 18 will be discharged.
A resistor 22 is coupled to a zener diode 24. The zener diode 24 further is coupled to the optical-coupler 20. The optical-coupler 20 further is coupled to the output terminal of the power supply to generate the feedback signal VFB at the feedback terminal FB. A rectifier 26 is coupled between a terminal of the secondary winding NS and the output terminal of the power supply. A filter capacitor 28 is coupled to the rectifier 26 and another terminal of the secondary winding NS.
FIG. 2 shows a circuit diagram of a traditional driving circuit 38 shown in FIG. 1. As shown in the figure, the traditional driving circuit 38 comprises a V-to-I converter 380 for generating a reference current IR in response to a reference voltage VR. The V-to-I converter 380 includes an operational amplifier 381, a resistor 382 and a transistor 383. The driving circuit 38 further comprises a plurality of current mirrors including transistors 384, 385, 386 for generating a charge current I385 and a discharge current I386 in response to the reference current IR. A switch circuit 387 includes switches 388, 389 for generating the driving signal VD. The switches 388, 389 can be the transistors. The switch 388 is coupled to receive a supply voltage VCC. The switch 389 is coupled to the ground. The input terminals of the inverter 390, 393 are coupled to the control circuit 35 to receive the switching signal VG. The output terminals of the inverter 390, 393 are coupled to a control switch 395 and the switch 389 respectively. The inverter 390 further is coupled to the supply voltage VCC and the ground. The inverter 393 further is coupled to the transistor 386 and the ground. The control switch 395 is coupled to the switch 388 and the ground.
The switch 388 is turned on by the charge current I385 according to the turn off of the control switch 395. The control switch 395 is turned off when the level of the switching signal VG is in high level. Meanwhile, the switch 389 is turned off. Once the switch 388 is turned on, the switch 388 outputs the driving signal VD that the level of the driving signal VD is in high level and the level of the driving signal VD is correlated with the level of the supply voltage VCC. The level of the driving signal VD is in low level when the level of the switching signal VG is in low level. Once the level of the switching signal VG is in low level, the control switch 395 is turned on to turn off the switch 388. Meanwhile, the switch 389 is turned on. Thus, the switch 389 outputs the driving signal VD that the level of the driving signal VD is in low level and the level of the driving signal VD is correlated with the ground.
A zener diode 397 is coupled between the switch 388 and the ground. The zener diode 397 is used to clamp the level of the driving signal VD to a constant level for protecting the transistor 12 of the power supply when the switch 388 is turned on to generate the driving signal VD and the level of the supply voltage VCC is higher than the constant level. However, once the level of the supply voltage VCC is higher than the constant level, the zener diode 397 is turned on and forms a low impedance current path for the charge current I385. Thus, the charge current I385 will flows into the ground. However, the charge current I385 is wasted. Therefore, reducing the charge current loss for power saving is requirement. The object of present invention is to provide a high efficiency driving circuit.